A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 2 June 2012
This has just recently come up in the Kotei, & has had our gaming group questioning its legality.
A friend in our game group was in a battle against one (and only one) Shosuro Aroru. During the battle, he targeted Aroru with one of his actions, wherein his opponent redirected it with Aroru's reaction, declaring "nothing" as his chosen card! He even stated that this was possible since even range attacks needs no target. 3 player judges actually ruled that this was in fact a legal action, though we still highly doubt this. Our friend did win the match, but has made us wary, since we're aware that targeting of range attacks is part of it's effect & not its cost.
So basically, my question is: Can Aroru's reaction choose "nothing" as a legal target? From our game group's understanding, it's not since there's no physical card that was chosen to become the new target.
Shosuro Aroru 4F/5C -/9G/0PH Scorpion Clan • Ninja • Duelist • Experienced 2 • Unique Will not attach non-Ninja Followers. Reaction: When an action would target a Ninja at Aroru's location: Choose another of your cards. The action targets it instead, if legal. After the action's resolution, you may move Aroru to a different location. Battle: Other players may not take Reactions until this action ends. Ranged 5 Attack.
Ben Higgins (rules official), 3 June 2012
This has just recently come up in the Kotei, & has had our gaming group questioning its legality.
A friend in our game group was in a battle against one (and only one) Shosuro Aroru. During the battle, he targeted Aroru with one of his actions, wherein his opponent redirected it with Aroru's reaction, declaring "nothing" as his chosen card! He even stated that this was possible since even range attacks needs no target. 3 player judges actually ruled that this was in fact a legal action, though we still highly doubt this. Our friend did win the match, but has made us wary, since we're aware that targeting of range attacks is part of it's effect & not its cost.
So basically, my question is: Can Aroru's reaction choose "nothing" as a legal target? From our game group's understanding, it's not since there's no physical card that was chosen to become the new target.
Shosuro Aroru 4F/5C -/9G/0PH Scorpion Clan • Ninja • Duelist • Experienced 2 • Unique Will not attach non-Ninja Followers. Reaction: When an action would target a Ninja at Aroru's location: Choose another of your cards. The action targets it instead, if legal. After the action's resolution, you may move Aroru to a different location. Battle: Other players may not take Reactions until this action ends. Ranged 5 Attack.
EDIT: To address the OP's question a bit more clearly.
"Nothing" is not a vailid choice.
Your opponent would have to choose a card, assuming there is another card to choose. Nothing in the situation described would allow them to ignore this effect. However, since Reactions are not bound by the Rule of Location, that card can be any card in play, even if that chosen card would not be a legal target for the original action.
The argument your opponent used to justify this (Range Attack optional targeting), doesn't factor in to this situation, as they are two completely separate and distinct mechanics, where applying the logic of one to the other should never be done.
(Aside- I've seen this trick used choosing a holding, a region, etc... and since the redirection effect only kicks in if the new target is legal, choosing one of those cards likely results in the redirection failing, but still allows you to use Aroru's movement, if he' still around after the first action finishes resolving.)